U.S. patent number 4,034,160 [Application Number 05/664,409] was granted by the patent office on 1977-07-05 for system for the transmission of speech signals.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Petrus Josephus Van Gerwen.
United States Patent |
4,034,160 |
Van Gerwen |
July 5, 1977 |
System for the transmission of speech signals
Abstract
A speech bandwidth compression system uses a first set of
filters to separate the speech signal into baseband (300-800 hz)
and highband (800-3200 hz) signals. The baseband (first formant or
fundamental pitch frequency) is transmitted directly, and is also
rectified to form harmonics which are mixed with the highband
signals in a second set of (paired digital) filters which form sine
and cosine forms (amplitude and phase information) of the harmonic
components, which are then multiplexed for transmission to a
receiver which provides the corresponding bandwidth expansion
system to reconstruct the original speech signal.
Inventors: |
Van Gerwen; Petrus Josephus
(Eindhoven, NL) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
19823393 |
Appl.
No.: |
05/664,409 |
Filed: |
March 5, 1976 |
Foreign Application Priority Data
|
|
|
|
|
Mar 18, 1975 [NL] |
|
|
7503176 |
|
Current U.S.
Class: |
704/201 |
Current CPC
Class: |
H04B
1/667 (20130101) |
Current International
Class: |
H04B
1/66 (20060101); G10L 001/00 (); H04B 001/66 () |
Field of
Search: |
;179/1SA,15.55R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Kemeny; E. S.
Attorney, Agent or Firm: Trifari; Frank R. Steckler; Henry
I.
Claims
What is claimed is:
1. A speech transmission system comprising a band compressor
including a baseband channel means for transmitting a baseband of
speech signals located in a lower speech frequency range, a
plurality of parallel band compressor channel means for band
compression of successive sub-bands of speech signals above said
lower range, means having an input coupled to said base-band
channel and an output coupled to said band compressor channels for
generating components in said sub-bands of said speech frequency
range, a plurality of parallel reference channel means for
generating reference frequency components in the same sub-bands as
said compressor sub-bands, at least one of said plurality of
channels comprising a frequency analyzer coupled to said component
generator and having for each sub-band first and second
sub-bandpass filters with the same amplitude frequency
characteristics and differing phase-frequency characteristics, each
of said band compressor channels including a comparator having a
first input means for receiving a constant amplitude signal from
said component generating means, a second input coupled to the
respective reference channel to receive said reference signal
components and an output means for providing phase and amplitude
data characteristics of the respective sub-bands.
2. A transmission system as claimed in claim 1, wherein the
frequency analyser having for each sub-band a first and a second
sub-bandpass filter with the same amplitude-frequency
characteristics having a relative phase difference comprises a
delay circuit and a plurality of weighting networks, one end of
each of these weighting networks being connected to a point of a
respective delay time in the delay circuit, and a summing circuit
coupled to the remaining ends.
3. A transmission system as claimed in claim 2, wherein the first
and second sub-bandpass filters for the same sub-band in the
frequency analyser have phase-frequency characteristics having a
relative phase difference of .pi./2.
4. A transmission system as claimed in claim 1, wherein the
comparator in the band compressor comprises a first and a second
modulator, said modulators being controlled by the frequency
components of constant amplitude from the component generator and
by the frequency components in the relevant sub-band of the
original speech signal, a pair of low pass filter means coupled to
said modulators respectively for providing the phase and amplitude
data of the relevant sub-band.
5. A transmission system as claimed in claim 1, wherein the band
compressor further comprises a consonant detector fed by the speech
signals to be transmitted, a noise generator, an electronic switch
means controlled by the consonant detector for connecting the
component generator in response to a vowel and for connecting said
noise generator in response to a consonant to the input of the
plurality of band compressor channels.
6. A transmission system as claimed in claim 1, wherein the
base-band channel in the band compressor and a baseband channel in
a receiver band expander together comprise a synchronization
circuit for the mutual synchronization of the component generators
in the band compressor and in the band expander.
7. A transmission system as claimed in claim 1, wherein the
remaining plurality of channels further comprises an additional
frequency analyser having only one sub-bandpass filter for each
sub-band.
8. A transmission system as claimed in claim 1, further comprising
a limiter circuit means for obtaining the constant amplitude value
of the frequency components including separate limiters which are
connected to the sub-bandpass filters of the frequency
analyser.
9. A transmission system as claimed in claim 8, wherein the
baseband signal comprises the first formant frequency range, and
the component generator connected to the baseband channel comprises
a frequency distorter which includes a non-linear circuit.
10. A transmission system as claimed in claim 1 further comprising
a limiting circuit means for obtaining the constant amplitude value
of all frequency components comprising a common limiter in the
component generator.
11. A transmission system as claimed in claim 10, wherein the
component generator comprises a fundamental tone pulse
generator.
12. A transmission system as claimed in claim 4, wherein the
component generator comprises a fundamental tone pulse generator,
the output signal of the fundamental tone pulse generator being
supplied direct to the two modulators of the comparator, said
modulators comprising switching modulators.
13. A transmission system as claimed in claim 12, further
comprising a delay circuit coupled between the fundamental tone
pulse generator and the comparator and having a delay time equal to
the delay time of a frequency analyser in a receiver band
expander.
14. A transmission system as claimed in claim 4, further comprising
a common limiter disposed in the component generator to obtain a
constant amplitude value of all frequency components, the original
speech signal being supplied direct to two modulators of the
comparator, said modulators comprising product modulators.
15. A receiver for band compressed speech signals comprising a band
expander including a baseband channel, a plurality of a sub-band
expander channels, means having an input coupled to said baseband
channel and an output coupled to said expander channels for
generating components in said subbands, a frequency analyzer
coupled to said component generator and having for each sub-band
first and second sub-bandpass filters with the same
amplitude-frequency characteristics and differing phase frequency
characteristics, and a modulator circuit having a first input means
for receiving a constant amplitude signal from said component
generator, a second input means for receiving reference phase and
amplitude data, and an output means for supplying a portion of the
original speech signal.
16. A receiver as claimed in claim 15 wherein the modulator circuit
in the band expander comprises a first and a second modulator, said
modulators being controlled by frequency components of constant
amplitude from the component generator and by the phase and
amplitude data belonging to the relevant sub-band, and a summing
circuit means connected to the modulators for providing the
recovered sub-band.
17. A receiver as claimed in claim 15 characterized in that the
frequency analyser comprises a delay circuit and a plurality of
weighting networks, one end of each of these weighting networks
being connected to a point of a respective delay time in the delay
circuit, and a summing circuit coupled to the remaining end.
18. A receiver as claimed in claim 17, wherein the first and second
sub-bandpass filters for the same sub-band in the frequency
analyzer have phase-frequency characteristics having a relative
phase difference of .pi./2.
19. A receiver as claimed in claim 15, wherein the band expander
includes a noise generator, electronic switch means controlled by
the baseband channel for connecting the component generator in
response to a vowel and for connecting said noise generator in
response to a consonant to the input of the plurality of band
expander channels.
20. A receiver as claimed in claim 15, wherein a base-band channel
in a transmitter band compressor and the base-band channel in the
band expander together comprise a synchronization circuit for the
mutual synchronization of the component generators in the band
compressor and in the band expander.
21. A receiver as claimed in claim 15, further comprising a limiter
circuit means for obtaining the constant amplitude value of the
frequency components including separate limiters which are
connected to the sub-bandpass filters of the frequency
analyser.
22. A receiver as claimed in claim 21, wherein the baseband signal
comprises the first formant frequency range, and the component
generator connected to the baseband channel comprises a frequency
distorter which includes a non-linear circuit.
23. A receiver as claimed in claim 15, further comprising a
limiting circuit means for obtaining the constant amplitude value
of all frequency components comprising a common limiter in the
component generator.
24. A receiver as claimed in claim 23, wherein the component
generator comprises a fundamental tone pulse generator.
Description
The invention relates to a system, for speech signal transmission
provided with a transmitter having a band compressor and with a
receiver having a band expander, the band compressor at the
transmitter comprising a baseband channel for a baseband of speech
signals located in a lower speech frequency range and also a set of
parallel band compressor channels for band compression of
successive sub-bands of the speech signals in the speech frequency
range located above the baseband, the band expander at the receiver
comprising a baseband channel and a corresponding set of parallel
band expander channels for recovering the original speech signals.
The first formant frequency range or the fundamental tone frequency
may, for example, be used as a baseband signal.
In a known transmission system of the above-mentioned type, each of
the parallel band compressor channels comprises a frequency divider
and an amplitude detector for generating, by means of frequency
division and amplitude detections frequency and amplitude data in
respect of each of the subbands of the speech signals. The
frequency and amplitude data obtained in this manner is
transmitted, together with the first formant range which serves as
the baseband signal. At the receiver, each of the parallel band
expander channels comprises a frequency multiplier and an amplitude
modulator for recovering, by means of frequency multiplication and
a subsequent amplitude modulation, the original subbands of the
speech signals from the frequency and amplitude data in the
corresponding band compressor channel, whereafter the original
speech signal is obtained by combining these recovered sub-bands
with the first formant range serving as the baseband signal.
It is an object of the invention to provide another concept of a
transmission system of the above-mentioned type which combines a
considerable improvement of the reproduction quality with a hardly
critical and conveniently structured implementation and which is
particularly suitable for use in digital techniques and integration
in a semiconductor body.
The transmission system according to the invention is characterized
in that the band compressor and the band expander each comprise an
input circuit which is provided with a component generator
connected to a baseband channel for generating frequency components
in the sub-bands of the speech frequency range located above the
baseband and in that the band compressor and the band expander each
further comprise a frequency analyser having for each sub-band a
first and a second sub-band filter with the same
amplitude-frequency characteristics but with phase-frequency
characteristics having a relative phase difference, the band
compressor together with the set of parallel band compressor
channels also comprising a series of parallel reference channels,
at least one of these channel set including said frequency analyser
having for each sub-band a first and a second sub-band filter,
which band compressor includes in the various band compressor
channels a comparator as well as a limiting circuit for obtaining a
constant amplitude value of the frequency component generated in
the component generator at the input of the comparator which is
controlled through a reference channel by frequency components in
the same sub-band of the orifinal speech signal, which frequency
components serves a reference signal for generating phase and
amplitude data characteristic of the relevant sub-bands, said
frequency analyser having per sub-band a first and second sub-band
filter included in the band expander in the set of parallel band
expander channels, the various band expander channels also
comprising a modulator circuit as well as a limiting circuit for
obtaining a constant amplitude value of the frequency components
generated in the component generator at the input of the modulator
circuit which is controlled by the characteristic phase and
amplitude data of the relevant sub-band.
The invention and its advantages will now be further explained with
reference to the figures.
FIG. 1 and FIG. 2 show, an embodiment of a transmitter and of a
receiver in a transmission system according to the invention;
FIG. 3 and FIG. 4 show, a variant of the transmitter and of the
receiver shown in FIG. 1 and FIG. 2;
FIGS. 5-8 show in more detail some elements which are used in the
transmission system having the transmitter and receiver of FIG. 3
and FIG. 4; and
FIG. 9 shows a simplified embodiment of a transmitter in a
transmission system according to the invention.
The transmitter and receiver shown in FIG. 1 and FIG. 2 are part of
a transmission system which is arranged for the transmission of
speech signals by means of pulse-code modulation.
For the transmitter shown in FIG. 1, the speech signals in the
frequency band of from 300-3200 Hz are derived from a microphone 1
and are supplied, after amplification in an amplifier 2, to a band
compressor 3 which is provided with a baseband channel 4 for a
baseband signal located in a lower speech frequency band which in
the embodiment shown is formed by the first formant range of
300-800 Hz. The band compressor 3 is also provided with a series of
parallel band compressor channels 5 for band compression of
successive sub-bands of the speech signals in the frequency range
800-3200 Hz located above the baseband. In the figure only one of
the parallel band compressor channels is illustrated in detail as
these channels are exactly the same in structure.
The baseband channel 4 is provided with an input filter 6, which
passes the first formant range and which has a passband of 300-800
Hz, and a subsequent amplifier 7. The output of baseband channel 4
as well as the outputs of the sub-band compressor channels 5 are
connected to a time division multiplex distributor 8 which feeds a
subsequent pulse code modulator 9. In this way, a pulse train
characterizing the speech signals to be transmitted is produced at
the output of the pulse code modulator 9, which train is
transmitted through a transmission path 11 after amplification in
an output amplifier 10 having a suitably dimensioned output
filter.
In the receiver in FIG. 2, which is associated with the transmitter
shown in FIG. 1, the pulse train transmitted via the transmission
path 11 is supplied, after pulse regeneration in a pulse
regenerator 12 and subsequent demodulation in a pulse code
demodulator 13, to a time division multiplex distributor 14 which
is connected to a band expander 15 for recovering the original
speech signals. Like the band compressor 3, the band expander 15 is
provided with a baseband channel 16 having an input filter 17
passing the baseband signal of 300-800 Hz and a subsequent
amplifier 18. This band expander 15 is also provided with a series
of band expander channels 19 corresponding with the series of band
compressor channels 5. The outputs of the baseband channel 16 and
of the band expander channels 19 are connected to a summing circuit
20.
From the summing circuit 20 the recovered original speech signals
in the band from 300-3200 Hz are derived and are fed to a
reproducing device 22 via an amplifier 21.
In accordance with the invention a considerable improvement in the
reproduction quality is obtained because the band compressor 3 in
the transmitter of FIG. 1 comprises an input circuit which is
provided with a component generator 23 connected to the baseband
channel 4 for generating frequency components in the sub-bands of
the speech frequency range of 800-3200 Hz which are located above
the baseband of 300-800 Hz, and the band compressor 3 comprises
furthermore a frequency analyzer 24 having for each sub-band a
first sub-band filter 25 and a second sub-band filter 26 with the
same amplitude-frequency characteristic but with phase-frequency
characteristics mutually shifted in phase, (digital filters 25 and
26 are taught in U.S. Pat. No. 3,521,170), the band compressor 3
together with the sequence of parallel band compressor channels 5
also comprising a series parallel reference channels 32, and at
least one of the channel sequences 5, 32 including the above said
frequency analyser 24 having for each sub-band a first and a second
sub-band filter 25 and 26, which band compressor 3 includes in the
various band compressor channels 5 a comparator 31 as well as a
limiting circuit 29, 30 for obtaining a constant amplitude value of
the frequency components generated in the component generator 23 at
the input of the comparator 31, which is controlled through a
reference channel 32 by frequency components in the same sub-band
of the original speech signal, which frequency components serve as
a reference signal, for generating phase and amplitude data
characteristic of the relevant sub-band.
In the embodiment shown the frequency analyser 24 is included in
the series of parallel band compressor channels 5, the sub-band
filters 25, 26 with outputs 27, 28 are each connected through a
separate limiter 29, 30 to the comparator 31, and the component
generator 23 is constructed as a signal distorter including a
non-linear circuit. In FIG. 1 the frequency analyser 24 is formed
by a delay circuit 33 and a plurality of weighting networks 34 . .
. 35; 36 . . . 37 one end of each of which is connected to a point
having a respective delay time in the delay circuit 33 and the
other end to a summing circuit 38; 39 which is connected to one of
the outputs 27, 28.
When the transfer factors of the weighting networks 34 . . . 35; 36
. . . 37 are suitably dimensioned, splitting up of the speech
frequency range of 800-3200 Hz in successive sub-bands in
accordance with a desired amplitude-frequency characteristic and
phase-frequency characteristic can be realised in a simple manner
and with a large mutual freedom by means of the frequency analyser
24 described, as will now be mathematically explained. If the
number of delay elements of the delay circuit 33 is equal to 2M,
each element having a delay time s, and if in a given sub-band
filter, for example the sub-band filter 25, the weighting networks
34 . . . 35 starting from the ends of the delay circuit 33 have
been equalled pairwise, so that the transfer coefficients C.sub.p
satisfy:
then a transfer function is obtained whose amplitude-frequency
characteristic has the form .psi. (.omega.): ##EQU1## and whose
phase-frequency characteristic .phi. (.omega.) is exactly linear
according to:
Thus, the amplitude-frequency characteristic constitutes a Fourier
series developed in M cosine terms, whose periodicity .OMEGA. is
given by:
If a given amplitude-frequency characteristic .psi..sub.o (.omega.)
must be realised, then the coefficients C.sub.p in the Fourier
series can be determined by means of the equation: ##EQU2##
Negative coefficients C.sub.p in the Fourier series can be realised
by including a polarity reversing stage in series with the
weighting networks.
The form of the amplitude frequency-characteristic is herewith
fully determined, but the periodic behaviour of the Fourier series
results in that the desired amplitude frequency characteristic
repeats itself with a periodicity .OMEGA. = 2 .pi./s, so for
sufficiently low values of the delay time s the frequency spacing
between the desired and the next additional passband can be made
sufficiently large to suppress the additional passbands by means of
a simple suppression filter without affecting the
amplitude-frequency characteristic and the linear phase-frequency
characteristic in the desired passband in a noticeable manner. To
that end, a simple suppression filter in a form of an RC low-pass
filter may for example, be included in the summing circuits 38,
39.
Although other dimensioning rules are possible, it has proved to be
advantageous for the construction described of the sub-band filter
25 to approximate the transfer characteristic of the associated
second sub-band filter 26 with the same amplitude-frequency
characteristic but with a phase-frequency characteristic which is
shifted in phase, preferably with a phase shift .pi./2, by a series
having M sine terms. In order to distinguish them, the transfer
coefficients of the weighting networks 36 . . . 37 are indicated
herein by S.sub.p. Starting from the ends of the delay circuit 33
the transfer coefficients S.sub.p of the weighting networks 36 . .
. 37 are pairwise equal in magnitude but of opposite sign, whilst
the transfer coefficient of the central weighting network S.sub.o
is equal to zero so that the transfer coefficients S.sub.p of the
weighting networks satisfy the equation:
for the transfer function then applies: ##EQU3## From the above
formulae it appears that the sub-band filter 26, like the sub-band
filter 25, has a linear phase-frequency characteristic which,
however, as compared with that of the sub-band filter 25 shows a
constant phase shift of .pi./2, and that the amplitude-frequency
characteristics of the sub-band filters 25, 26 can be made mutually
equal by a suitable dimensioning of the coefficients S.sub.p in the
Fourier series which may be determined by means of the equation:
##EQU4##
Through the limiters 29, 30, the sub-band filters 25, 26 are
connected to the comparator 31 which is composed of modulators 40,
41 and subsequent low-pass filters 42, 43 having cut-off
frequencies of, for example, 20 Hz. At the same time, the phase and
amplitude reference formed by the frequency components in the same
sub-band of the original speech signal is fed to the two modulators
40, 41 through the relevant reference channel 32. In the embodiment
of FIG. 1 a frequency analyser 44 is also included in the series of
reference channels 32, which analyser is connected by means of an
input filter 45 having a pass-band of 800-3200 Hz to the output of
amplifier 2. The frequency analyser 44 comprises a number of
sub-band filters 46, which correspond with the number of speech
sub-bands and is constructed in like manner as the frequency
analyser 24 above described from delay circuit 47, a plurality of
weighting networks 48 . . . 49 and a summing circuit 50: the
dimensioning rule of the sub-band filter 25 is applied with
advantage for the weighting networks 48 . . . 49.
By comparing the limited frequency components of the sub-band
filters 25, 26 with the phase and amplitude reference of the
frequency analyser 44 in the modulators 40, 41 and by the
subsequent smoothing in the low-pass filters 42, 43, the phase and
amplitude data associated with the sub-band of the band compressor
channel 5 shown in FIG. 1 is obtained in the comparator 31, which
data is supplied to the time division multiplex distributor 8 for
further transmission together with the phase and amplitude data of
the remaining band compressor channels (not shown in FIG. 1) and
the baseband signal.
In the transmitter described, the first formant range of the speech
signal in the band of 300-800 Hz is supplied as a baseband signal
on the one hand to the baseband channel 4 through the amplifier 7
and on the other hand to the component generator 23 which is
constructed as signal distorter, and thus at any instant, by means
of signal distortion, the speech frequency components in the band
800-3200 Hz are artificially derived from the baseband signal of
300-800 Hz, which components are individually selected in the
various sub-band filters 25, 26 by means of a suitable dimensioning
of the frequency analyser 24. For this purpose the successive
passbands of the sub-band filters 25, 26 have, for example, been
made equal to 800-880 Hz, 880-960 Hz, 960-1040 Hz, 1040-1120 Hz, .
. . , respectively. By means of signal distortion in signal
distorter 23, the frequency spectrum of the speech signals is
generated in the band of 800-3200 Hz at any instant in a proper
approximation. After having been brought to a constant amplitude in
the limiters 29, 30, the frequency components selected in the
sub-band filters 25, 26 are then compared, in the modulators 40,
45, with associated output filters 42, 43 of comparator 31, in
amplitude and phase with the exact frequency components of the
speech signal located in the band of 800-3200 Hz which is fed to
the frequency analyser 44 through the input filter 45. No special
demands need be made on the frequency analyser 24, as the selected
frequency components are brought to a constant amplitude by the
limiters 29, 30 independent of the amplitude-frequency
characteristic of the sub-band filters 25, 26.
The various frequency components in the band compressor channels 5
are not only characterized by an amplitude and a frequency datum
but also, in a clear contradistinction with the above-mentioned
transmission system, by an additional phase datum which enables a
more exact recovery of the various frequency components and which
consequently results in a considerable improvement in the
reproduction quality. Surprisingly, this improvement in the
reproduction quality owing to the additional phase datum does not
entail complications as regards equipment, but on the contrary it
is accompanied by a simple and moreover convenient structure of the
band compressor channels 5 for generating the phase and amplitude
data as will now be explained mathematically.
To this end sub-band k of the speech signal will be considered. If
it is assumed that at a given instant a frequency component of the
original speech signal is selected by the sub-band filter 46 of the
frequency analyser 44, which component may be represented by
b.sub.k cos.omega..sub.k t, b.sub.k being an amplitude and
.omega..sub.k the instantaneous angular frequency, and if it is
furthermore assumed that the corresponding frequency component
generated in the signal distorter 23 has a phase error .phi..sub.k,
then after frequency selection in the sub-band filters 25, 26 and
limiting to a constant amplitude in the limiters 29, 30, the
signals cos (.omega..sub.k t+ .phi..sub.k) and sin (.omega..sub.k
t+ .phi..sub.k) will be obtained (next to higher harmonics) at the
output of the limiters 29, 30. For simplicity it is here assumed
that limitation was effected at a unit amplitude.
By means of modulation the signals
b.sub.k cos .omega..sub.k t cos (.omega..sub.K t+ .phi..sub.k)
b.sub.k cos .omega..sub.k t sin (.omega..sub.k t+ .phi..sub.k),
are then respectively produced at the output of the two modulators
40, 41 from which, by means of smoothing with the aid of the
low-pass filters 42, 43, the amplitude and phase data are obtained
in the form of b.sub.k cos .phi..sub.k and b.sub.k sin .phi..sub.k
which are fed to the time division multiplex distributer 8 for
transmission by means of pulse-code modulation through the
transmission path 11 for recovering the original speech signals in
the receiver. It is precisely the use of the amplitude and phase
data in the form of b.sub.k cos .phi..sub.k and b.sub.k sin
.phi..sub.k which results in the particularly simple and little
critical structure of the described band compressor 3. The same
applies to the band expander 15 at the receiver side as will be
explained with reference to the associated receiver in FIG. 2.
Compared with the transmission of speech signals in the band of
300-3200 Hz without band compression by means of pulse code
modulation for which 60 kbit/s is used, the present transmission
system only requires 10 kbit/s owing to the use of the band
compressor 3 described, which corresponds with a band compression
factor of 6. Besides the advantages of an excellent reproduction
quality already mentioned and a simple, little critical and
conveniently arranged structure the system described also has the
advantage that it is particularly suitable for implementation in
digital techniques and so for integration in a semiconductor
body.
It should be noted that instead of the embodiment in FIG. 1 in
which two sub-band filters 25, 26 having phase-frequency
characteristics mutually shifted in phase are used for each
sub-band in the frequency analyser 24, and only one sub-band filter
46 in frequency analyser 44, and other embodiment is also possible
in which, to obtain the amplitude and phase data b.sub.k cos
.phi..sub.k and b.sub.k sin .phi..sub.k, the frequency analyser 24
is provided with only one sub-band filter for each sub-band and the
frequency analyser 44 with two sub-band filters having
phase-frequency characteristics mutually shifted in phase.
In the receiver of FIG. 2, the data of the band compressor 3 of
FIG. 3, which is transmitted by means of pulse code modulation
through transmission path 11, becomes available, after pulse-code
demodulation, in the pulse-code demodulator 13, for recovering the
original speech signals in the band expander 15 at the outputs of
the time division mutliplex distributor 14, particularly the first
formant range of 300-800 Hz as the baseband signal for the baseband
channel 16 and at leads 51, 52 the phase and amplitude data b.sub.k
cos .phi..sub.k and b.sub.k sin .phi..sub.k of the band compressor
channel 5 for sub-band k.
In accordance with the invention the band expander 15 comprises in
FIG. 2 an input circuit which is provided with a component
generator 53 connected to the baseband channel 16 for generating
frequency components in sub-bands of the speech frequency range of
800-3200 Hz which is located above the baseband of 300-800 Hz, and
the band expander 15 comprises furthermore a frequency analyser 54
having for each sub-band a first sub-band filter 56 and a second
sub-band filter 57 with the same amplitude-frequency characteristic
but with phase-frequency characteristics mutually shifted in phase,
which frequency analyser 54 is included in the series of parallel
band expander channels 19, and the various band expander channels
19 include a modulator circuit 55 as well as a limiting circuit 60,
61 for obtaining at the input of modulator circuit 55 a constant
amplitude value of the frequency components generated in the
component generator 53, which modulator circuit 55 is controlled by
the characteristic phase and amplitude data of the relevant
sub-band available at leads 51, 52.
As in the band compressor 3 of FIG. 1, the component generator 53
of FIG. 2 is constituted by a signal distorter and the outputs 58,
59 of the sub-band filters 56, 57 are connected each to a separate
limiter 60, 61, which here, however, are connected to the modulator
circuit 55 which is composed of modulators 62, 63 whose outputs are
connected to a summing circuit 64; optionally a simple output
filter 65 may be arranged after the adder circuit 64. Like the
frequency analyser 24 at the transmitter side, the frequency
analyser 54 is composed of a delay circuit 66, a plurality of
weighting networks 67 . . . 68; 69 . . . 70 and summing circuits
71, 72, and the dimensioning of the sub-band filters 56, 57 has
also been made equal to that of the sub-band filters 25, 26.
Also, in this case, the recovery of the original speech signals is
based on the artificial generation of the speech frequency
components in the band of 800-3200 Hz by distorting the baseband
signal in the band of 300-800 Hz in the signal distorter 53. By
selection in the sub-band filters 56, 57 and limiting in the
limiters 60, 61, the frequency components for the various band
expander channels 19 are obtained, from which components the
sub-bands of the original speech signals are recovered by means of
modulation in the modulator circuit 55 with the amplitude and phase
data b.sub.k cos .phi..sub.k and b.sub.k sin .phi..sub.k of the
leads 51, 52, which sub-bands are combined in the summing circuit
20 connected to the reproduction device 22 through the amplifier
21. Speech signals of an excellent reproduction quality are
produced at the reproduction device 22, as the frequency components
which were already generated with a good approximation in the
signal distorter 53 are, at the proper frequency position,
furthermore brought into accurate agreement as regards phase and
amplitude with the original speech frequency spectrum by the band
expander channels 19 described, as will now be mathematically
explained.
Because the band compressor channels 5 and the band expander
channels 19 comprise the same signal processing circuits which are
formed by a signal distorter 23; 53, sub-band pass filters 25, 26;
56, 57, limiters 29, 30; 60, 61, the same output signal which is
represented above by cos (.omega..sub.k t+ .phi..sub.k) and sin
(.omega..sub.k t+ .phi..sub.k) will also be derived from the
limiters 60, 61 in the band expander channel 19 for sub-band k at
the instant of reception which corresponds with the considered
instant of transmission. At the same time the associated amplitude
and phase data b.sub.k cos .phi..sub.k and b.sub.k sin .phi..sub.k
is presented through leads 51, 52 to the modulators 62, 63 in the
relevant band expander channel 19. In this way a signal s(t) will
be obtained at the output of the summing circuit 64 in the
modulator circuit 55 which signal has the form:
which may be reduced to:
So as regards frequency, phase and amplitude this signal s(t) is
exactly in agreement with the relevant spectrum component in the
original speech signal, which therefore guarantees a substantially
perfect reproduction quality.
Besides simplicity and a conveniently arranged structure the large
degree of agreement between the band compressor channels 5 and the
band expander channels 19 is apparent: all elements of the band
expander channels 19, the summing circuit 64 and possibly the
simple output filter 65 excepted, are already present in the band
compressor channels 5 at the transmitter side, which from a
technological point of view is particularly advantageous as the
transmission system described can be realised with a minimum of
different elements. Moreover, the transmission system described is
suitable for implementation in digital techniques for integration
in a semiconductor body. In particular, the frequency analysers 24,
44, 54 can also be constructed (as an alternative to analog
techniques using an analog delay circuit composed e.g., of
inductors and capacitors or of capacitor shift register sections)
using digital techniques, for example in the manner which is
already described in the U.S. Pat. No. 3,521,170, and also the
modulators 40,41; 62, 63 used are particularly suitable for digital
implementation owing to the preceding limiters 29, 30; 60, 61. For
completeness sake it is to be noted that in the practical
embodiments the limiters 29, 30; 60, 61 can be combined with the
modulators 40, 41; 62, 63.
A variant of the transmission system discussed above is shown in
FIG. 3 and FIG. 4, which system differs from the transmission
system shown in FIG. 1 and FIG. 2 in that instead of the first
formant range the fundamental tone frequency is transmitted as the
baseband signal. Corresponding elements in FIGS. 1-4 are indicated
with the same reference numerals.
In the transmitter shown in FIG. 3 the speech signals in the band
of 300-3200 Hz are therefore supplied, after amplification in the
amplifier 2, to a fundamental tone detector 73 which supplies a
sinusoidal output signal of fundamental tone frequency in the band
of 80-250 Hz. The fundamental tone detector 73 is connected to the
amplifier 7 of baseband channel 4 and also to the component
generator 74 which is constituted by a fundamental tone pulse
generator, which component generator supplies, in the rhythm of the
fundamental tone in the original speech signal, a pulse shaped
output signal of the shape illustrated by curve 75 at a repetition
frequency in the band of 80-250 Hz.
As in the transmitter of FIG. 1, the component generator 74 in the
form of a fundamental tone pulse generator artificially generates a
good approximation of the frequency spectrum of the original speech
signal. After frequency selection in the two sub-band pass filters
25, 26 of the frequency analyser 24 the frequency components are
processed in the band compressor channels 5 to generate the phase
and amplitude data of the form b.sub.k cos .phi..sub.k and b.sub.k
sin .phi..sub.k at the output of the low-pass filters 42, 43 in
comparator 31.
The structure of this band compressor 3, however, differs from that
in FIG. 1 in that the constant amplitude value of the frequency
components (selected in the frequency analyser 24) at the input of
comparator 31 is not effected here by incorporating separate
limiters 29, 30 in the various band compressor channels 5 but by
incorporating a limiter in the component generator 74 constructed
as fundamental tone pulse generator, which limiter brings all
frequency components for the various sub-bands to a constant
amplitude value so that here a common limiter for all band
compressor channels 5 is sufficient. In exactly the same way as in
FIG. 1 also, the said phase and amplitude data of the form b.sub.k
cos .phi..sub.k and b.sub.k sin .phi..sub.k is derived in
comparator 31 from the various band compressor channels 5 by
comparing the phase and amplitude of the frequency components
selected in the frequency analyser 24 and brought to a constant
amplitude value with the corresponding frequency components of the
original speech signal selected in the sub-band pass filters 46 of
the frequency analyser 44. This data is transmitted together with
the baseband signal to the associated receiver in FIG. 4 through
the time division multiplex distributor 8, the pulse code modulator
9 and the amplifier 10. Compared with the transmission system in
FIG. 1 and FIG. 2, a larger band compression factor is obtained
with the transmission system now under consideration because,
instead of faithfully transmitting the speech signals in the first
formant range of 300-800 Hz, these speech signals are also
transmitted with band compression in this instance, for which the
bandwidth of the baseband signal need not be more than 170 Hz.
For completeness' sake, FIG. 5 and FIG. 6 show simple embodiments
of a fundamental tone detector 73 and a fundamental tone pulse
generator 74 which may be used in FIG. 3.
For the fundamental tone detector 73 shown in FIG. 5 the speech
frequencies in the band of 300-800 Hz derived from a filter 76 are
fed to an amplitude detector 77 which is followed by an output
filter 78 having a passband of 80-250 Hz, and a sinusoidal output
signal of fundamental tone frequency is derived from the output
filter 78 and fed through a lead 84 to the fundamental tone pulse
generator 74 shown in FIG. 6.
The fundamental tone pulse generator 74 shown in FIG. 6 comprises a
limiter 85 which is connected to the lead 84 and which is followed
by a differentiating network 86 for converting the square wave
voltage obtained by the limiting action into a train of alternating
positive and negative pulses, which network 86 is followed by a
threshold circuit 87 which, for example, suppresses the negative
pulses. Thus, a train of positive pulses of fundamental tone
frequency and of the form shown by curve 75 is produced at the
output of the threshold circuit 87 and fed to the frequency
analyser 24 for further processing.
As an alternative to the fundamental tone detector 73 and
fundamental tone pulse generator 74 shown in FIGS. 5 and 6,
fundamental tone detectors and fundamental tone pulse generators of
another type may be used in the transmission system according to
FIG. 3 and FIG. 4. For example, a fundamental tone pulse generator
may be used which supplies sawtooth output pulses, whilst a
bistable or monostable relaxation generator may be used as limiter
85 in FIG. 6.
In the receiver in FIG. 4 associated with the transmitter in FIG.
3, the incoming pulse train is fed, after pulse regeneration in a
pulse regenerator 12 and demodulation in a pulse code demodulator
13, to the time division multiplex distributor 14 which, for
recovering the original speech signals, is connected to the band
expander 15 provided with parallel arranged band expander channels
19 and with a baseband channel 16, to which a component generator
is connected which is controlled by the transmitted output signal
of the fundamental tone detector 73 and which is constituted by a
fundamental tone pulse generator 89.
Like the transmission system described in FIG. 1 and FIG. 2, an
accurate frequency and phase synchronisation is effected between
the frequency components in the output signals of the component
generator 74 of the band compressor 3 and those in the output
signals of the component generator 89 of the band expander 15 with
the aid of the synchronisation circuit formed by the baseband
channels 4, 16. The output signal of the component generator 89 is
processed in the same way as in the band expander 15 of FIG. 2. In
particular, the component generator 89 is constituted by the
fundamental tone pulse generator connected to the frequency
analyser 54 which comprises for each sub-band the sub-bandpass
filters 56, 57 whose outputs 58, 59 are connected to the modulators
62, 63 in the modulator circuits 55 controlled by the phase and
amplitude data of the form b.sub.k cos .phi..sub.k and b.sub.k sin
.phi..sub.k present on leads 51, 52. At the output of the modulator
circuits 55 the speech component b.sub.k cos .omega..sub.k t is
then recovered which, as explained in detail for FIG. 2, accurately
corresponds as regards frequency, phase and amplitude with the
relevant component in the original speech signal, which recovered
speech component is combined with the speech components of the
other band expander channels 19 in summing circuit 20 and supplied
to the reproduction device 22 through the amplifier 21.
Due to the limiting operation in the band compressor channels 5 and
the band expander channels 19 the transmission system described
also appears to be little critical in implementation. In particular
the proper operation is hardly affected by the form of the output
signal of the component generator 74 and 89, provided only the
generated frequency spectra, as regards their frequencies, form a
good approximation of the frequency spectrum of the original speech
signal, which fact gives more scope for designing and ultimately
results in a simplification of equipment. For example, instead of
the sawtooth output signal of the usual fundamental tone
generators, in the embodiment described the pulses of fundamental
tone frequency delivered by the fundamental tone pulse generator
shown in FIG. 6 will do for further processing.
As compared to FIG. 1 and FIG. 2 a considerable saving in equipment
is obtained in this embodiment because the separate limiters in the
various band compressor channels 5 and in the band expander
channels 18 are here replaced by a limiter which is common for all
band compressor channels 5 and band expander channels 19 in the
fundamental tone pulse generator 74, 89, and furthermore the
frequency analyser 44 in the band compressor 3 can be dispensed
with by using purely product modulators for modulators 40, 41.
Also, with the transmission system considered here, a very good
reproduction quality is realised, and, moreover, the reproduction
quality of the consonants can be improved here such as, for
example, the letter sounds "s" and "f" which are mainly
characterized by a noise-like frequency spectrum located in the
higher speech frequency range.
To this end, both the band compressor 3 in FIG. 3 and the band
expander 15 in FIG. 4 comprise a respective noise generator 90, 91
provided with a respective output filter 92, 93 having a suitable
passband, of, for example 300-3200 Hz, and a subsequent electronic
switch 94, 95 which comprises a switching contact 96, 97 controlled
by a switching signal and two contacts which are connected to
respectively the fundamental tone pulse generator 74, 89 and the
output filter 92, 93. Optionally, a limiter may be connected to the
output of the noise generator 90, 91 to suppress noise peaks.
Jointly the switching contacts 96, 97 of the two electronic
switches 94, 95 are controlled by a switching signal derived from a
consonant detector 98 connected to the amplifier 2 in the
transmitter, namely the electronic switch 94 in the band compressor
3 through a lead 99 and the electronic switch 95 in the band
expander 15 through a lead 100, time division multiplex distributor
8, time division multiplex distributor 14 at the receiver side and
a lead 101. Besides a saving in equipment the joint control by the
same consonant detector 98 results in an accurate synchronisation
of the switch-over instants of the electronic switches 94, 95 in
the band compressor 3 and the band expander 15.
For illustration, FIG. 7 shows in more detail a simple embodiment
of the consonant detector 98 in FIG. 3. This consonant detector is
composed of two parallel branches 102, 103 having respectively a
filter 104 which passes the lower speech frequencies of, for
example, 300-800 Hz and a filter 105 which passes the higher speech
frequencies of, for example, 2000-3000 Hz, which filters are
followed by rectifiers 106, 107 and by low-pass filters 108, 109
having a cut-off frequency of, for example, 20 Hz for generating
output signals which vary with the level of said speech
frequencies. After amplification in an adjustable amplifier 110,
111 these signals yield in an amplitude comparator 112 a switching
signal, the polarity of which, for example, depends on whether a
vowel in the form of, for example, the letter sounds "a" and "e",
or a consonant in the form of, for example, the letter sounds "s"
and "f" are produced. Namely, a suitable adjustment of the
amplification of the amplifiers 110, 111 results in that, depending
on whether a vowel or a consonant is produced, the signal level of
the lower speech frequencies or that of the higher speech
frequencies will dominate in the amplitude comparator 112.
If a vowel is produced then the switching contact 96, 97 of the
electronic switches 94, 95 in the band compressor 3 and in the band
expander 15 will be adjusted by the switching signal to the
position where it is connected to the fundamental tone pulse
generator 74, 89. In the manner explained hereinbefore the phase
and amplitude data of the various speech components is then
generated in the comparator 31 of the various band compressor
channels 5 and by means of this data the various components of the
original speech signal are recovered in the modulator circuit 55 of
the corresponding band expander channels 19, which components are
supplied to the reproduction device 22 through summing circuit 20
and amplifier 21. In the reverse case, when a consonant is
produced, the switching contact 96, 97 of the electronic switches
94, 95 will be adjusted by the switching signal of the consonant
detector 98 to the position where it is connected to the noise
generators 90, 91. In the comparator 31 of the various band
compressor channels 5 the noise data is then generated and in the
modulator circuit 55 of the various band expander channels 19 the
correct amount of noise for the consonant reproduction will be
realised by means of this noise data.
In this simple manner an excellent reproduction of both vowels and
consonants is guaranteed. In the transmission system according to
FIG. 1 and FIG. 2 the noise character at the reproduction of
consonant may possibly be accentuated in the manner described
here.
In a variant of the transmission system shown in FIG. 3 and FIG. 4,
a fundamental tone frequency detector is used for a further
bandwidth reduction instead of the fundamental tone detector 73
which enables said further bandwidth reduction by means of
frequency detection of the fundamental tone. If, for example, the
fundamental tone used as the baseband signal still occupies the
band of 80-300 Hz, then the frequency-detected fundamental tone
only occupies a band having a width of 20 Hz.
For illustration FIG. 8 shows a simple embodiment of a fundamental
tone frequency detector. It comprises a fundamental tone detector
having, as in FIG. 5, an input filter 76, an amplitude detector 77
and an output filter 78 as well as a subsequent frequency detector
79, the detected fundamental tone being supplied in the manner as
in FIG. 6 for frequency detection, to the cascade arrangement of a
limiter 85, a differential network 86, and a threshold circuit
87.
As in FIG. 6, a train of pulses of fundamental tone frequency and
of, for example, a positive polarity is produced at the output of
the threshold circuit 87. By smoothing this pulse train in a
lowpass filter 83 having a cut-off frequency of, for example, 20 Hz
an output signal is obtained which varies with the fundamental tone
frequency, which signal is supplied to the fundamental tone pulse
generator in the band compressor 3 and through the baseband channel
4 to the fundamental tone pulse generator in the band expander
15.
The fundamental tone pulse generator associated with the
fundamental tone frequency detector in FIG. 8 differs from the
fundamental tone pulse generator in FIG. 6 in that instead of a
limiter 85 an astable relaxation generator of the multivibrator
type having a natural frequency of 165 Hz is used, which relaxation
generator is controlled in its frequency by the output signal of
the fundamental tone frequency detector in FIG. 8. A square-wave
output signal which varies with the fundamental frequency is
produced at the output of the relaxation generator, which signal is
further processed through the differentiating network 86 and the
threshold circuit 87 in the same way as in FIG. 6.
Besides the special advantages in structure and implementation
specified above, this variant of the transmission system also has
the advantage that a maximum band compression factor is realised,
while maintaining a good reproduction quality.
FIG. 9 shows a particularly simple embodiment of the transmitter
shown in FIG. 3 for a transmission system according to the
invention. Elements in FIG. 9 which correspond with FIG. 3 are
given the same reference numerals.
Compared with FIG. 3, the transmitter in FIG. 9 differs in that the
component generator formed by the fundamental tone pulse generator
74 as well as the noise generator 90 are here connected through the
electronic switch 94, without the use of the frequency analyser as
an intermediate circuit, to the two modulators 40, 41 of the
comparator 31 in each of the parallel band compressor channels 5. A
further distinction is that now a frequency analyser 113 having for
each sub-band a first sub-bandpass filter 114 and a second
sub-bandpass filter 115 with the same amplitude-frequency
characteristic, but with phase-frequency characteristics having a
relative phase difference, are included in the series of parallel
reference channels 32. The frequency analyser 113 is constructed in
exactly the same manner as the frequency analyser 24 already
described above and is, in particular, provided with a delay
circuit 116, a plurality of weighting networks 117 . . . 118; 119 .
. . 120 and summing circuits 121, 122 having outputs 123, 124 which
are connected to the modulators 40, 41 of comparator 31.
In its operation this band compressor 3 in FIG. 9 fully corresponds
with that in FIG. 3. In greater detail, when a vowel is produced,
the phase and amplitude data of the relevant band compression
channel 5 of the form b.sub.k cos .phi..sub.k and b.sub.k sin
.phi..sub.k is generated by comparing the pulses of fundamental
tone pulse generator 74 in the modulators 40, 41 with the reference
signals obtained from sub-bandpass filters 114, 115 and, when a
consonant is produced, the noise data belonging to the relevant
consonant is generated by connecting the noise generator 90 through
the electronic switch 94 to modulators 40, 41.
In the manner set forth above the various data of the band
compressor 3 is transmitted to a band expander 15 of the type shown
in FIG. 4 for recovering the original speech signals. An accurate
phase synchronisation of the band expander 15 with the band
compressor 3 can be realised in a simple manner by using the band
compressor described here by, for example, including at the output
of the electronic switch 94 a delay circuit having a delay time
equal to the delay time of the frequency analyser 54 in the band
expander 15.
As in the transmission system of FIG. 3 and FIG. 4, an excellent
reproduction quality is realised when the band compressor 3 shown
in FIG. 9 is used, and together with the advantage that a frequency
analyser can be dispensed with, the additional advantage is
obtained that the modulators 40, 41 may be implemented as switching
modulators which are particularly suitable for integration in a
semiconductor body.
Within the scope of the invention still further embodiments are
possible. In FIG. 1 and FIG. 2 it is, for example, possible to
utilize the cascade arrangement of a fundamental tone detector 73
and fundamental tone pulse generator 74 of the type shown in FIGS.
5 and 6 instead of the component generator 23, 53 in the form of a
signal distorter. In the band expander 15 in FIG. 4 it is
optionally also possible to supply the output signal of the
electronic switch 95 direct to modulators 62, 63, it then being
necessary that the frequency analyser with sub-band filters having
the same amplitude-frequency characteristics, but with
phase-frequency characteristics showing a relative phase difference
is arranged behind the modulators 62, 63.
* * * * *